Environmental Engineering Reference
In-Depth Information
3.4.2
Anaerobic Digestion
Anaerobic digestion (AD) involves a mixture of bacteria including syntrophic
bacteria, fermentative bacteria, acetogenic bacteria and methanogenic bacteria
to decompose biomass under anaerobic conditions in order to produce biogas
(methane and hydrogen) as fuel. The process is divided into four stages:
hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In the hydrolysis
stage, insoluble organic compounds are broken down into water-soluble mono-
mers by hydrolases secreted by a consortium of bacteria. The barely decom-
posable polymer, which is not decomposed by hydrolases, remains solid and
thus limits the efficiency of AD. The products of hydrolysis are then converted
into short-chain organic acids, alcohols, aldehydes, and carbon dioxide in the
acidogenesis stage and transformed into acetates, carbon dioxide, and hydro-
gen in the acetogenesis stage. Finally, methanogenic bacteria utilize acetic
acid, hydrogen, and carbon dioxide to produce methane. About 70% of the
methane is converted from acetic acid while the remaining 30% results from
carbon dioxide reduction. Biogas and digestate, which is the remaining solids
with the mineralized nutrient, are the final products [85].
The biogas composition varies with substrates due to the composition differ-
ence. Lipids give the highest methane yield (1,014 m
3
ton
-1
) while the methane
yields of carbohydrates and proteins are much lower (415 and 496 m
3
ton
-1
, respec-
tively). Figure 3.15 depicts the yield of methane of different substrates [86].
Figure 3.16 shows the process flow diagram of the Hong Kong Organic Waste
Treatment Facilities Phase II, which is an AD plant [87]. This facility can process
Used grease
800
Baking wastes
714
Fish oil
680
Old bread
600
Food residue
240
Molasse
190
Maize
200
Wheat straw
131
Sudan grass
125
Grass
102
Cow manure
30
Pig manure 5
0
100
200
300
m
3
methane/ton of waste
400
500
600
700
800
900
Figure 3.15
Methane yield from different substrates [86].
